Genetic differences in avoidance behavior: Cardiovascular activity, pain sensitivity and stress-induced analgesia

Four experiments are reported which examine cardiovascular activity, pain sensitivity and stress-induced analgesia in rats selectively bred for differences in shuttlebox avoidance behavior. The results of Experiments 1 and 2 indicate that the two genetic lines differ in basal pain sensitivity, as measured by the hot-plate test. This difference in pain sensitivity appears not to be mediated by endogenous opioids, because it was not altered by pretreatment with a large dose of naloxone. In Experiments 1, 3 and 4 tail-flick tests of basal pain sensitivity failed to reveal line differences. Basal and stress levels of cardiovascular activity also showed no differences between the lines. In Experiment 3, LA but not HA animals showed profound stress-induced analgesia which was not blocked by a large dose of naltrexone. In Experiment 4, both LA and HA animals showed stress-induced analgesia, perhaps because the procedure of this experiment permitted conditioning mechanisms to contribute to the analgesia. Differential genetic selection for avoidance behavior also selected for differential pain sensitivity and some forms of non-opioid stress-induced analgesia but without concomitant selection for differential cardiovascular activity.